The present invention relates generally to a system and method for detecting small hole defects in materials.
Recently, material suppliers have been under increasing pressure for built-in quality and zero defect production programs for the materials they supply. Traditionally, small holes or defects in strips of material, such as sheet metal, have been difficult to find. Although pin hole inspection systems have existed for some time, these traditional systems are designed to detect holes in the micron range and are vital to the production and inspection of materials used in the manufacturing of products such as food containers and medical supplies. Unfortunately, these systems are expensive and difficult to operate in many industrial environments. Line scan camera surface inspection systems are currently being developed to detect various types of defects, including small holes. Like the earlier pin hole detector systems, they are expensive and difficult to install effectively in many industrial situations. Such known systems have often proved unreliable. One small hole in an otherwise perfect material can cost thousands of dollars in lost production.
The small hole detection system of the present invention is designed to overcome many of the problems associated with small hole defect inspection technology. The present invention is cost effective, easy to install, substantially maintenance free, and capable of detecting small holes in fast moving strips of material. The present invention delivers highly reliable service in the industrial environments where it is needed most.
Traditional hole detect systems had to be manually or mechanically adjusted to the width of the strip being scanned. This slowed the scanning process and required additional human intervention. The present invention contains a unique auto adjustment feature which allows the system to automatically adjust to the width of the strip being scanned.
Additionally, traditional hole detect systems utilized photomultiplier devices which were very expensive and unreliable. The present invention detects small holes without the use of photomultiplier devices thus reducing cost and increasing reliability.
Traditional hole detect systems have used high power lamps which require high energy considerations. The present invention utilizes LEDs which are used to scan the material strips. These LEDs are arranged and pulsed in a unique arrangement. Since the LEDs may not tolerate continued application of the high energy needed for small hole detection, the LEDs of the present invention are pulsed at a predetermined frequency to preserve the life of the LEDs. Pulsing at a predetermined frequency also makes stray light from the ambient environment a non-factor in the operation of the invention.
In summary, the present invention has many advantages over traditional hole detection systems, including:
1) detects holes or defects as small as 0.05" (inches) (1.27 mm); PA1 2) operational on lines with speeds as high as 3000' (feet) per minute; PA1 3) solid state reliability; PA1 4) no moving parts, therefore, less likely to wear out; PA1 5) dust, mist, and vibration tolerant; PA1 6) easy to install, simple to maintain; PA1 7) no light sources to replace; PA1 8) low voltage system with no high voltage necessary; PA1 9) auto-shuttering feature; PA1 10) not sensitive to ambient light; and PA1 11) relatively inexpensive.
These and other objects of the invention, as will be apparent herein, are accomplished by the hole detection system of the present invention which in one preferred embodiment comprises:
an array of LEDs; an array of photocells positioned directly across from the array of LEDs for detecting light emitted from the array of LEDs; a preamplifier circuit electrically connected to the photocells; a video processing circuit, electrically connected to the preamplifier circuit for processing a video signal from the preamplifier circuit and where the video processing circuit outputs an appropriate video signal when a hole is detected in the moving sheet product; a processing unit electrically connected to the video processing circuit for processing a video signal from the video processing circuit, and where the processing unit indicates when a hole in the moving sheet material has been detected.
It is preferred that the video processing circuit be comprised of: a sharply tuned bandpass filter electrically connected to the preamplifier circuit for filtering the video signal from the preamplifier circuit; a sample and hold amplifier electrically connected to the output of the preamplifier circuit; a comparator, electrically connected to the output of the sample and hold amplifier for filtering noise; and where the comparator switches to a higher voltage when the video signal from the preamplifier circuit exceeds the threshold voltage of the comparator.
It is also preferred that the processing unit be comprised of: a hole detect circuit electrically connected to the video processing circuit for triggering a hole detect signal when a hole is detected on the moving sheet material; and a "Fail-Safe" circuit electrically connected to the hole detect circuit for insuring that the hole detection system is scanning properly.
It is preferred that the processing unit be further comprised of: an edge counting circuit electrically connected to the hole detect circuit for counting edges detected in a scan; and a cylindrical lens which allows for closer spacing of the array of LEDs to the array of photocells to accommodate limited space requirements and to improve the signal to noise ratio of the hole detection system.
It is also preferred that the array of LEDs be comprised of: a first section of 50 LEDs; a second section of 50 LEDS; where these first and second sections of 50 LEDs be comprised of five groups of 10 LEDS; and where the first and second sections of LEDs are pulsed at a 20 kHz repetition rate.